A. Field of Invention
The present invention relates to controlling a high power alternating source for an electrostatic precipitator.
B. Background Art
It is known in the art to control an AC energy source for electrostatic precipitators using silicon control rectifiers (SCR). See for example Laugesen U.S. Pat. Nos. 4,326,860 and 4,390,830.
In this prior art a turn-on signal was applied to the gate of the SCR to turn the SCR on, usually after the peak of a half-cycle, and energy was applied to the electrodes of the electrostatic precipitator by way of the SCR. See, for example, the prior art waveform shown in FIG. 2E or SCR Manual, Fifth Edition, General Electric Company, Chapter 9 (AC Phase Control).
Since the current through an SCR must be decreased to substantially zero to turn the SCR off, after the SCR was turned on energy was supplied to the electrodes for the remainder of the half-cycle during which the SCR was turned on. Thus the SCR could control the energy from one end of the AC half-cycle only in the direction indicated by the arrows of FIG. 2E.
The SCR was usually turned on after the peak of a half-cycle because arcing of the electrodes is most likely at the peak of the AC signal. This delay in turning the SCR on avoided applying energy to the electrodes during the portion of the half-cycle most likely to cause arcing.
However, this also resulted in poor utilization of the waveform, since the portion of the half-cycle between a zero-crossing and a peak could not be applied to the electrodes. This was so because the turn-off time of the SCR was too long to turn an SCR on in the portion of the half-cycle before the peak and reliably turn it off before the peak to prevent arcing. See for example SCR Manual, Fifth Edition, General Electric Company, page 123 for a list of parameters which affect the turn off time of SCR's. Forced commutation circuits to accomplish this type of turn off were very complex and extremely expensive.
Additionally, the harmonic content and the DC ripple of the pulses produced in these SCR power supplies for electrostatic precipitators were objectionable when this arrangement was used because of the way that the DC waveform was chopped, especially with high current loads.
Furthermore, because it was difficult to turn off the SCR, it was difficult to terminate the supply of energy to the electrodes quickly under arcing or other emergency conditions. A further problem associated with shutdown upon arcing or other emergency shutdown was that this type of sudden shut-down a large amount of energy to be dumped into the precipitator, stressing precipitator components.
In addition to these difficulties, since the voltage rose during the early portions of the half-cycle before the SCR was turned on to supply current to the load, the voltage and current were out of phase resulting in a poor power factor.
It has also been known in the prior art to use gate turn-off thyristors (GTO) to operate from a DC voltage rail to obtain a variable frequency AC output. See for example, "Gate Turn-Off Thyristors: Their Properties and Applications", W. Bosterling, H. Ludwig, R. Schimmer, M. Tscharn; AEG-Telefunken, Primary Technical Information, October, 1983. However, this method was not useful for ESP technology because it would have to be applied to the energy supply after step-up and rectification where the voltage level is in the range of one hundred to two hundred kilovolts.